1. Field of the Invention
This invention relates to a heat mode recording and erasing method which is effected by applying thermal energy or light to a recording medium, and more particularly to a recording and erasing method which utilizes the reversible changes in the crystalline state of a recording layer comprising the optical anisotropic thin-film-shaped crystals of an organic material as caused by the application of thermal energy thereto. The present invention also relates to a recording medium employed in the above information recording and erasing method.
2. Discussion of Background
Recently, a recording system, referred to as a "heat mode recording system", has been developed. In this system, information is recorded in a recording material by applying light thereto which is converted to thermal energy so as to change the shape or physical properties of the recording material.
As the recording media for the heat mode recording system, inorganic recording media comprising as the main component a metal such as Te, Bi, Se, Tb or In, and organic recording media comprising any of organic dyes such as polymethine dyes, for example, cyanine; macrocyclic aza-azulene dyes, for instance, phthalocyanine, naphthalocyanine and polyfirryne; naphthoquinone dyes; anthraquinone dyes; and diol metal complex dyes, have been widely known as the heat mode recording media.
When thermal energy is applied to any of the above recording media by application of a laser beam, a recording layer of the recording medium is fused or vaporized, resulting in the formation of pits for information recording. However, the thus recorded information in the recording medium is not erasable, and new information cannot be overwritten on the recorded information. In other words, the heat-mode optical recording medium can be used only for recording information and reproducing the recorded information.
Attention is currently focused on a reversible recording medium in which not only recording and reproduction of information, but also erasing of recorded information can be achieved.
Magneto-optical recording media which comprise a thin alloy layer consisting of an alloy of rare earth elements such as Gd, Tb and Dy, and transition metals such as Fe, Ni and Co have been known as reversible recording media.
In this type of recording media, both thermal energy generated by application of a laser beam, and a magnetic field applied thereto are employed in combination for recording information. The recorded information is reproduced by using the difference in the rotation angle of the light reflected by the recording medium between a recorded area and a non-recorded area, which corresponds to the direction of the magnetization of the recording medium. The recorded information can be erased by applying an external magnetic field which is in the reverse direction to the magnetic field applied for recording information.
The magneto-optical recording medium, however, has a low reproduction sensitivity and an insufficient S/N ratio because of the small difference in the rotation angle of the light reflected thereby between a recorded area and a non-recorded area. Furthermore, the recording sensitivity of the recording medium is deteriorated with time, and recorded information cannot be stably stored in the recording medium for a long period of time due to the oxidization of the recording layer.
In addition to the above, a reversible recording medium which comprises a recording layer made of a thin layer of an inorganic material comprising as the main component an inorganic material such as Ge, Te, Se, Sb, In or Sn has also been known. In this recording medium, the phase transition between a crystalline state and an amorphous state in the recording layer is utilized for recording, reproducing and erasing of information. In this type of recording medium, recording information and erasing recorded information can be performed by mere application of a laser beam thereto in a heat mode. The recorded information is reproduced by detecting the difference in reflectance between a recorded area and a non-recorded area in the recording medium. However, the difference in reflectance between the recorded area and non-recorded area of the recording medium is so small that there are still problems with the reproduction speed and the reliability.
Japanese Laid-Open Patent Applications 54-119377, 55-154198, 63-39378 and 63-41186 disclose resin matrix materials and thermal recording materials comprising any of the resin matrix materials and finely-divided particles of a low-molecular weight organic material dispersed in the matrix material. Any of these thermal recording materials becomes opaque (in a light shielding state) when it is heated to a certain temperature or above and then cooled. When the recording material is heated to another certain temperature, and then cooled, it becomes transparent. The above phenomenon is reversible, so that both recording and erasing of information can be achieved. The contrast obtained by the above-mentioned light-shielding property is sufficient for visual reading. However, the contrast is insufficient for the observation of a portion with a size of several micrometers in a recorded area which is to be microscopically enlarged. This is because the finely-divided particles of a low-molecular weight organic material are dispersed in the resin matrix in the recording layer, so that light scattering in the recording layer changes, depending upon the state of the lower-molecular weight organic material. However, when the recorded area is in the size of several micrometers, the size of the finely-divided particles is too small to cause the particles to scatter light. In order to avoid this problem, it may be considered that the recording layer is made much thicker than the thickness required for recording. However, in practice, it is extremely difficult to heat the thick recording layer uniformly in the direction of its thickness to form a small recorded area. Thus, the above-mentioned recording media are not suitable for high density recording.
Other erasable recording media and recording methods which use organic materials are proposed. For instance, mixtures of an organic metal complex bis(1-p-n-alkylphenylbutane-1,3-dionato)(II) and organic polymeric materials are disclosed in Japanese Laid-Open Patent Application 58-199343; mixtures of crystalline thermoplastic resins and amorphous thermoplastic resins in Japanese Laid-Open Patent Application 63-15793; crystalline aromatic vinylene sulfide polymers in Japanese Laid-Open Patent Applications 63-95993 and 63-96748; diazabicyclo[2,2,2]-octane quaternary salts, and stretched polymers, and recording methods of using the crystallization of liquid crystalline polymers, amorphous transition and changes in orientation of polymers in Japanese Laid-Open Patent Application 63-259851. These recording media and recording methods cannot be employed in practice because the recording and erasing speeds are slow.